A source of compact ultrafast pulses, generating pulses that are transform-limited with a duration approaching a single optical cycle, is in high demand for such applications as ultrafast spectroscopy, fluorescence spectroscopy, photochemistry and photophysics, coherent controlled micro-spectroscopy, multiphoton microscopy, fluorescence lifetime imaging, and non-linear biomedical imaging. White-light generation by pumping an optical fiber pumped with an oscillator-type ultrafast laser is a promising technology for this source.
The coupling of a pulsed pump laser into a (longitudinally) uniform optical fiber has allowed nonlinear conversion of pump wavelengths into wavelengths that are difficult to access, while the pursuit of various favorable conditions has contributed to a basic theoretical understanding of soliton and dispersive wave dynamics. Notably, the 1999 discovery of Ti:sapphire oscillator-induced octave-spanning continuum, described in Ranka et al., Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm, Opt. Lett., vol. 25, pp. 25-27 (2000), stimulated the theoretical tools of reduced Maxwell's equations which attributed this phenomenon to the fission of higher-order solitons into red-shifted fundamental solitons and their blue-shifted phase-matched dispersive waves. The resulting unusual prediction that long pump pulses would be advantageous over short (fs) ones for uniform spectral broadening has led, for example, to the combination of a picosecond Yb:fiber master-oscillator-power-amplifier (MOPA) with a dispersion-engineered photonic crystal fiber (PCF), described by Travers, in Blue extension of optical fibre supercontinuum generation, J. Opt., vol. 12, 113001 (2010). In the Yb:fiber MOPA platform, however, the simple soliton-dispersive-wave picture must incorporate a pulse trapping mechanism to synchronize the blue and red expansions of the continuum, of the sort described by Gorbach et al., Light trapping in gravity-like potentials and expansion of supercontinuum spectra in photonic crystal fiber, Nat. Photon., vol. 1, pp. 653-, (2007), incorporated herein by reference. This condition achieves broad spectrum and high spectral brightness, but at the cost of temporal coherence due to high soliton order (N) modulation instability.
A complementary translation of the Yb:fiber MOPA results to a femtosecond Er:fiber platform using specifically Ge-doped fibers has lowered the soliton order, and thereby improved the coherence of the output, as described by Krauss et al., Synthesis of a single cycle of light with compact erbium-doped fibre technology, Nature Photon., vol. 4, pp. 33-36 (2009), incorporated herein by reference. Despite its narrower bandwidth and lower spectral brightness, the supercontinuum based on an Er:fiber platform allows few-cycle pulse compression and single-cycle pulse synthesis. Taking advantage of all-fiber pumps, the picosecond Yb:fiber platform and the femtosecond Er:fiber platform have arguably become the most popular techniques for fiber supercontinuum generation.
The use of a Ti:sapphire amplifier, at 800 nm, to produce energetic deep-UV pulses (down to 200 nm) has been demonstrated by Joly et al., Bright spatially coherent wavelength-tunable deep-UV laser source using an Ar-filled photonic crystal fiber, Phys. Rev. Lett., vol. 106, 203901 (2011) (hereinafter, “Joly (2011)”), incorporated herein by reference. This effect has been explained not only by the reduced Maxwell's equations but also, equally well, by the generalized nonlinear Schrödinger equation (GNLSE) under the slowly varying envelope approximation, indicating that complex theoretical models may not be necessary in sub-cycle regime modeling, as shown by Joly (2011).
Technology for upconversion of infrared pulses was described by the current inventors in U.S. Pat. No. 8,305,682 (issued Nov. 6, 2012, hereinafter “Tu '682), which is incorporated herein by reference. Methods described therein produced ultrashort pulses shifted from the infrared into the visible portion of the spectrum by nonlinear mechanisms such as four-wave mixing or by Cherenkov radiation (CR), otherwise referred to herein as resonant dispersion wave (RDW) generation.
RDW generation was predicted in 1986, by Wai et al., Nonlinear pulse propagation in the neighborhood of the zero-dispersion wavelength of monomode optical fibers, Opt. Lett., vol. 11, pp. 464-66 (1986), and has often been referred to as “Cherenkov radiation” in the literature. Visible RDW has only been observed in highly nonlinear fibers, and has been limited to an average power of a few mW, beyond which a dramatic spectral transformation toward continuum generation occurs at rather short (10 cm) fiber lengths, as shown by Hu et al., Frequency-tunable anti-Stokes line emission by eigenmodes of a birefringent microstructure fiber, Opt. Express, vol. 12, pp. 1932-37 (2004).
Prior methods, however, have produced average pulse power on the order of only milliwatts at most. An increase of average pulse power available in generated ultrashort pulses by an order of magnitude or more is desirable for purposes of non-linear imaging, in the biomedical context or otherwise.
It would thus be desirable for some new principle to allow for a resonant dispersion wave to attain hitherto precluded levels of average power. Such a principle has been uncovered by the present inventors and is now described.